A wide variety of mental and physical processes are known to be controlled or influenced by neural activity in the central and peripheral nervous systems. For example, the neural-functions in some areas of the brain (i.e., the sensory or motor cortices) are organized according to physical or cognitive functions. There are also several other areas of the brain that appear to have distinct functions in most individuals. In the majority of people, for example, the areas of the occipital lobes relate to vision, the regions of the left interior frontal lobes relate to language, and the regions of the cerebral cortex appear to be consistently involved with conscious awareness, memory and intellect. The spinal cord is also organized so that specific regions of spinal cord are related to particular functions. Because of the location-specific functional organization of the central nervous system in which neurons at discreet locations are statistically likely to control particular mental or physical functions in normal individuals, stimulating neurons at selected locations of the central nervous system can be used to effectuate cognitive and/or motor functions throughout the body.
The neural activity in the central nervous system can be influenced by electrical and/or magnetic energy that is supplied from an external source outside of the body. Various neural functions can thus be promoted or disrupted by applying an electrical current to the cortex or other part of the central nervous system. As a result, the quest for treating or augmenting neural functions in the brain, spinal cord, or other parts of the body have led to research directed toward using electricity or magnetism to control these functions.
In several existing applications, the electrical or magnetic stimulation is provided by a neural-stimulator that has a plurality of therapy electrodes and a pulse system coupled to the therapy electrodes. The therapy electrodes can be implanted into the patient at a target site for stimulating the desired neurons. For example, one existing technique for masking pain in the lower extremities of a patient is to apply an electrical stimulus to a desired target stimulation site of the spinal cord. Although determining the general location of the target stimulation site may be relatively straight forward, identifying the specific configuration of electrodes for applying the stimulus will generally vary for specific patients.
The conventional procedure for optimizing the configuration of therapy electrodes involves several steps and relies on the subjective input from the patient. Conventional techniques generally involve rendering the patient unconscious, implanting an electrode array in the patient at the stimulation site, and then letting the patient regain consciousness. After the patient is conscious, the particular configuration of electrodes is optimized for that patient by selecting different combinations of the therapy electrodes and applying a constant electrical stimulus. The patient subjectively evaluates the effectiveness of each stimulus by indicating the degree to which the stimulus masks the pain. After testing the various configurations of therapy electrodes and deciding upon a desired electrode configuration according to the input of the patient, the patient is rendered unconscious for a second time to close the electrode array in the patient.
A similar procedure can be followed for determining the desired configuration of therapy electrodes for intra-cranial electrical stimulation. For example, a device for stimulating a region of the brain is disclosed by King in U.S. Pat. No. 5,713,922. King discloses a device for cortical surface stimulation having electrodes mounted on a paddle. The paddle can be implanted under the skull of the patient so that the electrodes are on the surface of the brain in a fixed position. King also discloses that the electrical pulses are generated by a pulse generator implanted in the patient remotely from the cranium (e.g., subclavicular implantation). The pulse generator is coupled to the electrodes by a cable that extends from the paddle, around the skull, and down the neck to the subclavicular location of the pulse generator.
King discloses implanting the electrodes in contact with the surface of the cortex to create paresthesia, which is a vibrating or buzzing sensation. More specifically, King discloses inducing paresthesia in large areas by placing the electrodes against particular regions of the brain and applying an electrical stimulus to the electrodes. This is similar to implanting therapy electrodes at the spinal cord of a patient for masking pain in the lower extremities of a patient, and thus King appears to require stimulation that exceeds the membrane activation threshold for a population of neurons at the electrodes (supra-threshold stimulation). King further discloses applying a stimulus to one set of electrodes, and then applying a stimulus to a separate configuration of electrodes to shift the location of the paresthesia.
One problem of the procedures for optimizing the configuration of therapy electrodes for either spinal or cortical stimulation is that existing systems and methods are expensive and time consuming. First, it is expensive to render the patient unconscious, implant the neural-stimulators in the patient, then wait for the patient to regain consciousness, then test various electrode configurations by asking the patient to subjectively estimate the degree to which the particular stimulus masks the pain, and then finally render the patient unconscious again to complete the implantation. Second, it can be a reasonably high risk operation because the patient is placed under general anesthesia at two separate stages of the process. It will be appreciated that this is an extremely long process that requires highly skilled doctors and personnel to attend to the patient for a significant period of time. Moreover, the patient occupies costly operating rooms and utilizes expensive equipment throughout the process. Third, relying on the subjective response from the patient may not provide accurate data for evaluating minor variances in the results. Fourth, the patient may experience pain or discomfort because some configurations may provide high intensity stimulation that exceeds the sensory level of stimulation. Therefore, existing systems for determining a desired configuration of electrodes to apply a neural-stimulus to specific patients are expensive, time consuming, potentially painful, and may not determine the most effective electrode configuration.
Another drawback of configuring the therapy electrodes using existing systems and methods is that the procedures are not effective for on-going use. This is because the patient's condition changes continually. For example, the location of the pain or the sensation typically shifts over time such that the optimal configuration of the electrodes at one point of the therapy may not mask the pain after a period of time. A large number of patients accordingly terminate electrical therapies for paresthesia within one year because of such a shift in the location of the pain/sensation. Therefore, although electrical stimulation for masking pain, inducing or enhancing plasticity, and other reasons appears to be very promising, it has not yet gained wide acceptance because of the drawbacks of configuring the therapy electrodes to apply an effective stimulus to different patients over a long period of time.
Additionally, it is also difficult to optimize the parameters of the electrical or magnetic stimulus. For example, even when a desired configuration of therapy electrodes is used, different waveforms can produce different results in each patient. Determining the stimulus parameters of the waveform can be even more time consuming than determining the desirable configuration of therapy electrodes because it involves testing a large number of independent variables. In a biphasic pulse train, for example, the stimulus parameters can include (a) the intensity of the electrical current, (b) the time of the stimulus of the first phase, (c) the time of the stimulus of the second phase, (d) the total time of the stimulus pulse, (e) the frequency of the stimulus pulse, (f) the pulse duty cycle, (g) the burst time of the stimulus, (h) the burst repetition rate of the stimulus, and (i) additional variables. Because of the large number of stimulus parameters, a particular waveform for the stimulus is typically selected for a given treatment for all patients such that the parameters for stimulus itself are not optimized.
In light of the several drawbacks for existing techniques of applying electrical or magnetic neural-stimulation to produce desired results, there is a significant need to enhance the procedures for applying such stimulus to individual patients. For example, it would be desirable to have more cost effective and less time consuming procedures for determining an effective configuration of therapy electrodes and stimulus parameters. Additionally, it would be desirable to update the electrode configuration and stimulus parameters in each individual patient without surgically operating on the patient to compensate for shifts in the target stimulation site.